The present invention relates to a precipitate hardening stainless steel having excellent structure stability, strength, toughness, and corrosion resistance, which requires no sub-zero treating and thus is excellent in terms of productivity, and a long blade for a steam turbine using the same.
In recent years, in light of energy saving (e.g., conservation of fossil fuel) and global warming prevention (e.g., reduction of CO2 gas generation), improvement of thermal power plant efficiency (e.g., improvement of steam turbine efficiency) has been anticipated. Elongation/enlargement of long blades for steam turbines is an effective means for improving steam turbine efficiency. In addition, elongation/enlargement of long blades for steam turbines allows reduction of the number of cylinders, which can be expected to result in additional effects such as shortening of facility construction time and subsequent cost reduction.
Materials for long blade having excellent mechanical properties and corrosion resistance are required in order to improve steam turbine reliability. Precipitate hardening stainless steels are characterized in that they are excellent in terms of corrosion resistance because they contain large amounts of Cr but small amounts of C; however, their strength and toughness are unbalanced (see, for example, JP Patent Publication (Kokai) No. 2005-194626 A).
Materials that contain increased amounts of precipitate-forming elements to achieve high strength have low martensite finish points (martensite finish temperatures). Such materials are problematic in terms of productivity because, for example, they should be subjected to sub-zero treating via dry ice cooling in order to realize uniform martensitic structure (see, for example, JP Patent Publication (Kokai) No. 2008-546912 A).